1. Field of the Invention
The present invention relates to an apparatus for correcting image blur caused by a camera shake and, more specifically, to an apparatus for correcting image blur in which image blur caused by camera shake can be corrected immediately after the opening of a shutter.
2. Description of the Related Art
It has been proposed to incorporate an angular velocity sensor in a video camera so as to detect atitude of the camera (Japanese Patent Laying-Open No. 61-289769). An angular velocity sensor such as an autogiro using a high-speed rotating body, or one utilizing Coriolis force generated from a vibrating tuning fork has been used. Now, image blur caused by a camera shake can be corrected if such an angular velocity sensor is incorporated in a camera and a portion of a taking lens of the camera is driven based on the output from the angular velocity sensor so as to offset the image blur. Precision in correcting image blur may be increased if a stepping motor is used for driving the portion of the taking lens.
Correction of image blur using the stepping motor will be described in the following.
FIG. 1 shows an image blur (displacement) caused by a camera shake. The displacement f.sub.F (mm) is shown as a track of image point existing at the center of the frame at the time t.sub.A. The periods A and B are exposure periods when photographing is done with shutter speed of 1/10 sec, with different timings of shutter opening. Time change of the image blur speed f.sub.F ' (mm/s), corresponding to the time change of the displacement f.sub.F shown as an example in FIG. 1, is shown in FIG. 2. As is apparent from the figure, in the exposure period A, the speed f.sub.F ' of image blur gradually decreases immediately after the opening of the shutter, while in the exposure period B, the speed f.sub.F ' of image blur gradually increases immediately after the opening of the shutter. In such a case, the stepping motor can follow the image blur in the exposure period B, however, the stepping motor can not always follow the image blur in the period A. The reason for this will be described in the following.
The speed-torque characteristic of the stepping motor is shown in FIG. 3. The speed of the stepping motor is represented as pulse speed (pulse number [PPS] inputted to the stepping motor per 1 second). In an self-starting region, start, stop and reverse rotation of the stepping motor can be done at a moment regardless of the combination of the pulse speed and a torque. However, in a through region, the stepping motor can not be started at once, so that slow up control becomes necessary, in which the motor is started in the self-starting region, the pulse speed is increased gradually, and then it is driven in the through region.
FIGS. 4 and 5 show examples of image blur correction in the exposure period A, showing relations between displacement, speed and time. In the hatched potion of FIG. 5, the stepping motor is not started, and it follows the input pulse from the time t.sub.S. The reason for this is that the pulse speed f.sub.P of the image blur correcting pulses applied at the start is high and is in the through region of the stepping motor. When the pulse speed f.sub.P of the image blur correcting pulses gradually decreases as time passes and enters the self-starting region, the motor starts at the time t.sub.S. Referring to the image displacement of FIG. 4 corresponding to the above operation, the actual correction amount f.sub.M to the time t.sub.s is 0, while the aimed amount of correction is (-f.sub.F), namely, correction shown by the hatched potion S.sub.X is not carried out. After the time t.sub.S, the actual amount of correction f.sub.M does not coincide with the aimed amount (-f.sub.F) of correction. However, the distance .DELTA. l between these amounts in the direction of displacement becomes constant, realizing the correcting realizing the correcting operation. Meanwhile, in the exposure period B, the pulse speed inputted to the stepping motor in the initial stage is in the self-starting region and the stepping motor is immediately started following the input pulse, so that correcting operation can be carried out throughout the exposure position B, as shown in the example of FIG. 6.
Therefore, in the exposure period A, correcting operation immediately after the opening of the shutter can not be done, while in the exposure period B, correcting operation can be carried out immediately after the opening of the shutter.
As described above, the stepping motor has two operation regions, that is, the self-starting region and the through region. When the stepping motor is started immediately after the opening of the shutter, control under the through region can not be realized, so that the possible speed of image blur correction is slower than the specific limit of the stepping motor.